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Really n00bish beginners' question about motion

  1. Sep 15, 2006 #1
    If for every action there is an equal and opposite reaction, then how come net forces exist?
    Surely when a force is exerted on an object, an equal force must push back (reaction) in the opposite direction, therefore balancing the forces, creating no net force and thus no acceleration?
    How do things move at all?
  2. jcsd
  3. Sep 15, 2006 #2


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    Because the reaction force acts on a different object.
  4. Sep 15, 2006 #3
    When walking, you exert a force on the sidewalk. In turn, the sidewalk exerts a force on you. The sidewalk doesn't pull and push you at the same time.
  5. Sep 15, 2006 #4
    I shall just echo the above posts:

    While every force does have an equal an opposite reaction force, these forces act on two different bodies.

    Think of a bullet leaving a gun: they both experience the same force in opposite directions; the much larger mass of the gun + dude with gun compared to the mass of the bullet means that the bullet flies off, but the gun recoils a bit (well not as much anyway).
  6. Sep 16, 2006 #5
    Ahh, the whole topic of mechanics makes sense now. :p
  7. Sep 16, 2006 #6


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    So, arguably, I suppose the OP is right when he questions the "net force". If he looks at the whole system - the bullet and gun, there is no net force. It is only if you look at a part of the system - just the nasty part of the bullet-gun combo - that you see a net force.
  8. Sep 16, 2006 #7
    Indeed. That's why momentum is conserved, when the whole system is considered.
  9. Sep 16, 2006 #8


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    Or to put it another way, if the net force on all parts of the system is zero, the center of mass of the system remains at rest, or continues moving in a straight line at constant speed. That is, the center of mass of the system obeys Newton's First Law, in this situation.
  10. Sep 17, 2006 #9
    well the reaction and action force act on two different bodies .when you are talking about net force you are only considering one body , but yes if you consider both bodies together the net result might be zero
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